The M.39B Libellula (from Libellulidae, a taxonomic family of dragonflies) was a Second World War tandem wing experimental aircraft built by Miles Aircraft; a scale version of the M.39 design proposed by Miles to meet Air Ministry specification B.11/41 for a fast bomber. The M.39B was used by Miles to generate data from which the M.39 design was improved but the M.39 project was cancelled and the B.39B broken up. Though it had some problems, the earlier M.35 proved sufficient to show the idea had merits, and the larger M.39 was drawn up as a twin-engined design prepared to meet Specification B.11/41 which had been issued by the Air Ministry for a high speed bomber.